The present invention relates to optical inspection apparatus and methods for inspecting the surface of a silicon wafer or other substrate. More particularly, the invention relates to an apparatus and methods for mapping the topography of a surface of a wafer or other substrate, and for identifying defect regions of the substrate based on a topographical map of its surface.
In a variety of applications it is desirable to be able to produce a map of the topography of a surface in situations where mechanical means for mapping the surface are impractical or inadequate. Particularly in the case of a highly polished surface having very small undulations or deviations from an ideal perfectly flat surface, mechanical measuring devices are generally incapable of providing the degree of resolution required to accurately map such a surface, and also may cause damage to the surface. Accordingly, non-contact means for mapping such a surface are preferred.
For instance, manufacturers of integrated circuits are constantly striving to reduce the line widths of conductors laid down on the surface of a silicon wafer to form integrated circuits. Currently, lines widths as narrow as 0.18 xcexcm are being used, and it can be expected that the line widths will continue to be reduced. Reduced line widths cause severe depth of focus limitations in the lithography process. Surface undulations, when combined with the reduced depth of focus, can cause the lines to be laid down inaccurately. Accordingly, the lithography process as well as other considerations create a need to minimize surface irregularities of the starting silicon wafer.
Because the surface quality of the wafer is an important ingredient in the quality of the resulting integrated circuits, inspection devices and methods are needed for detecting surface irregularities that might cause a defective circuit to be produced. It will be appreciated that even very slight deviations from a perfectly flat surface may be too large to be tolerated, particularly where the line widths being laid on the wafer are very small. Thus, there is a need for an apparatus and method capable of mapping with extremely fine resolution the surface topography of a highly smooth surface such as a silicon wafer surface. For example, it would be desirable to be able to map a wafer surface with a height resolution on the order of about 10 nm or less, and with a spatial resolution (in the in-plane direction of the wafer) on the order of about 200 xcexcm or less.
Optical inspection devices and methods have been developed for detecting the presence and sizes of defects in and on the surface of a polished substrate, for example for use in the production of silicon wafers, and defect sizes on the order of a few tens of nanometers can be detected. Among the known commercially available apparatus for mapping the topography of a wafer, however, the best height resolution that is achieved is about 20 nm or perhaps slightly less, and the best spatial resolution is on the order of several millimeters. There is currently no known apparatus available commercially that is capable of producing a complete map of the surface topography of a highly smooth surface such as that of a silicon wafer with a very fine spatial resolution (e.g., about 200 xcexcm or less) and an extremely fine height resolution (e.g., about 10 nm or less).
The present invention provides apparatus and methods enabling production of a complete topographical map (also referred to herein as a height map) of a highly smooth surface of a substrate such as a wafer, and for identifying defect regions of the surface based on a topographical map. Thus, in a preferred embodiment of the invention, a mapping apparatus comprises a light source adapted to produce an incident light beam and positioned to direct the incident beam to impinge on the surface and specularly reflect therefrom, a scanning system operable to move at least one of the substrate and the incident beam so as to move the incident beam in relation to the substrate such that the incident beam is impinged on the surface at a plurality of spaced-apart points to create a specularly reflected beam from each of the points, a light detector which receives each of the specularly reflected beams and provides a signal as a function of a change in location of each of the reflected beams relative to a reference location, and a processor which receives the signal corresponding to each of the points on the surface and calculates based on each signal a change in slope of the surface at each point relative to a reference slope which corresponds to the reference location of the reflected beam. The slope change information is then converted into a height map of the entire surface.
The invention further provides a method whereby defect regions on the wafer surface are identified based on a given height map defined by an array of grid points each having an associated height value, regardless of how the height map is derived. The method entails calculating, at each of the grid points of the height map, a change in surface height over a predetermined distance along the surface in a plurality of different in-plane directions of the wafer. The height change in each of the different directions is compared to a predetermined threshold. If any of the height changes in any of the directions exceeds the predetermined threshold, the grid point is identified as a defect region.
With respect to the apparatus aspect of the invention, the scanning system preferably is operable to scan the incident beam across the surface along a first direction such that the incident beam is reflected from an array of points on the surface that are spaced predetermined distances apart from each other along the first direction. The processor is operable to calculate changes in slope in the first direction for each of the points and to calculate relative surface heights of the points based on the changes in slope in the first direction. The scanning system similarly is operable to scan the beam across the surface in a second direction that is different from the first direction such that the beam is reflected from an array of points spaced apart along the second direction, thereby defining a two-dimensional grid of points. The first and second directions advantageously are perpendicular to each other, although they do not necessarily have to be so. For example, a skewed grid can be used in the present invention. Other geometrical arrays of points can also be used, as long as the array covers the whole wafer surface and adjacent points are spaced close enough together to provide the degree of spatial resolution that is desired. The scanning system can scan the beam in the first and second directions by keeping the substrate stationary and moving the beam, by moving the substrate and keeping the beam fixed, or by a combination of moving the substrate while also moving the beam.
In one preferred embodiment, the scanning system of the apparatus includes a transport mechanism operable to move the substrate so as to effect scanning of the beam over the surface. Advantageously, the transport mechanism is operable to translate the substrate along the second direction and the scanning system is operable to periodically scan the incident beam across the surface in the first direction. However, other devices and methods may be used for impinging the incident beam at a plurality of points on the surface, and the invention is not limited to any particular devices or methods for such purpose.
In accordance with a preferred embodiment of the invention, the light detector comprises a multiple-cell detector having a plurality of cells adjacently arranged, the detector providing a separate signal from each cell as a function of the amount of light intensity striking the cell. Thus, the relative strengths of the signals from the cells are indicative of the location on the detector at which the reflected beam strikes the cells. The processor receives the signals from the cells and calculates a change in location of the reflected beam based on the relative strengths of the signals. Preferably, the light detector comprises a quad-cell detector having four cells arranged in quadrants which are oriented such that the signals from the cells are indicative of surface slopes in two orthogonal X- and Y-directions along the surface. The processor preferably comprises a programmable microprocessor and is programmed to calculate surface slopes in the X- and Y-directions and to perform a line integration on the surface slopes to determine surface heights.